That's the worry about this Apollo Constellation engine.

At Bloomberg BusinessWeek, Ben Elgin writes a story with the scary title This Silicon Valley Space Startup Could Lace the Atmosphere With Mercury. He then describes a company that is "trying to develop better, cheaper propulsion systems for a new generation of satellites."

Apollo Fusion has designed its propulsion systems to use mercury as a fuel, according to four industry insiders with direct knowledge of its technology. NASA began moving away from mercury in the 1970s, owing to concerns about contamination on the ground.

Globe and Mail/ Lloyd Alter with ion drive engine/via

This is a subject that I know a little bit about; I built an ion drive engine when I was 17 and won a few science fairs with it. The mercury is not actually the fuel; this is an electric engine. It works like every other rocket: you throw something out the back at high speed and, thanks to Newton, that pushes your rocket forward. In this case, it's mercury, ionized and shot out at high velocity.

My engine was a pipe with a coil where the mercury vapour was ionized, then a series of mesh screens that attracted and accelerated the mercury ions and sent them on their way. It only works in a really serious vacuum; you can see the mercury diffusion pump behind the bell jar containing the engine. I never got to fire it up at the science fair because they thought I would poison everyone.

That's the problem. Mercury is really heavy and easily ionized, so it is the perfect propellant for an ion engine. But it is really toxic. Elgin goes through all of the things mercury does to people, how it is a powerful neurotoxin, how the USA has signed the Minimata treaty to reduce emissions.

Elgin goes on to do the math and figures that if Apollo launched a thousand satellites, each with 20 kg of mercury, it could put 20 metric tons of mercury into space, which he says would all eventually drift back into the atmosphere.